1. Field of the Invention
The present invention relates generally to a channel allocation method and apparatus in a broadband wireless communication system. More particularly, the present invention relates to a downlink channel scheduling method and apparatus that can obtain optimal system performance in a downlink of a wireless communication system using an orthogonal frequency division multiplexing (OFDM) scheme.
2. Description of the Related Art
Wireless communication systems using a multicarrier transmission scheme were first applied to military radio communications in the late 1950's. An orthogonal frequency division multiplexing (OFDM) scheme serving as a representative multicarrier transmission scheme for overlapping orthogonal subcarriers started to be developed in the 1970's. The OFDM scheme converts a serially input symbol stream into parallel signals and modulates the parallel signals using a plurality of orthogonal subcarriers to transmit the modulated parallel signals. The OFDM scheme has been widely used for digital data communication technologies such as digital audio broadcasting (DAB), digital television (TV) broadcasting, wireless local area network (WLAN), and wireless asynchronous transfer mode (WATM).
The OFDM system is suitable for a wireless communication environment that does not ensure line of sight (LOS), and is robust against multipath fading in a multipath environment, thereby providing an efficient platform for high-speed data transmission. That is, the OFDM system can efficiently overcome frequency selective fading by dividing an entire channel into narrowband orthogonal subchannels and transmitting the subchannels.
Moreover, the OFDM system can eliminate inter symbol interference (ISI) by adding, to a header of a symbol, a cyclic prefix (CP) with an interval longer than that of a delay spread interval of a channel. Accordingly, the OFDM system is the most effective way to transmit data at high speed. Due to this merit, Institute of Electrical and Electronics Engineers (IEEE) 802.16a was standardized, which is incorporated herein by reference. IEEE 802.16a supports multicarrier systems such as OFDM and orthogonal frequency division multiple access (OFDMA) systems as well as single-carrier systems.
The OFDMA system divides the frequency domain into subchannels comprising a plurality of subcarriers, divides the time domain into a plurality of time slots, and allocates a subchannel to each user. The OFDMA system is based on a multiple access scheme capable of accommodating multiple users using limited frequency resources by performing resource allocation while taking into account both the time and frequency domains.
The OFDMA system can assign a plurality of subchannels configured by different subcarriers to different users. When an adaptive antenna system (AAS) is used to increase system capacity in the OFDMA system, a subchannel can be configured by adjacent subcarriers. When the AAS is used, better channels are allocated to users in different channel environments, such that a multiuser diversity gain can be obtained.
When a multiple-input multiple-output (MIMO) system is used as a representative example of the AAS, information is spatially multiplexed and then the spatially multiplexed information is transmitted, such that communication system performance can be significantly improved. The system performance in the MIMO environment was verified by experiment and analysis. IEEE 802.16a-based OFDMA systems are classified into a time division duplex (TDD) system using the same frequency band between an uplink and a downlink, and a frequency division duplex (FDD) system using different frequency bands between the uplink and the downlink.
In the OFDMA/FDD system of the OFDMA systems, a base station (BS) must receive, from all active user terminals, feedback information comprising channel capacity information necessary for scheduling channels to be allocated. However, when the channel capacity information is received from all user terminals through the feedback information, there is a problem in that a system load of the BS significantly increases.